1. Field of the Invention
The present invention relates to an inverter apparatus for converting direct current (DC) to alternating current (AC) and outputting the same.
2. Description of the Prior Art
FIG. 12 shows the circuit diagram of a conventional inverter apparatus of this kind. In FIG. 12, Q.sub.1 and Q.sub.2 are switching elements driven by input signals from terminals A and B, respectively, comprising for instance transistors while Q.sub.3 and Q.sub.4 comprising switching elements connected to a primary coil N of an output transformer T, consisting of MOSFET, etc. The primary coil N of the output transformer T is provided with a center tap to which a DC power supply V.sub.cc is connected. C.sub.1 is a capacitor connected to the secondary coil S of the output transformer T. A load 1, e.g. CFL is connected through this capacitor C.sub.1. R.sub.1 and R.sub.2 are resistances.
The circuit of the foregoing configuration comprises an externally excited type inverter circuit; terminals A and B receive rectangular wave control signals from an external circuit consisting of a CPU or light control circuit, etc., continuously, which said control signals for instance comprising those of a frequency of several tens kHz with a phase shift of 180.degree.. With these control signals, switching elements Q.sub.3 and Q.sub.4 as connected to the primary coil N of the output transformer T also turn ON and OFF repeatedly, thus generating AC in the secondary coil S of the output transformer T. This AC is supplied to a load 1. At that time, a tank circuit is composed of the output transformer T and the capacitor C.sub.1. The resonant frequency of the tank circuit is matched with the frequency of control signals sent from said external circuit so that the output becomes maximum, thereby providing the load 1 with stabilized power. FIG. 13 shows the voltage waveforms at both terminals of the load 1.
However, with such a conventional inverter apparatus as described above, there is a drawback that the frequency of control signals entered from an external device cannot be matched exactly with the resonant frequency of the tank circuit comprising the output transformer and the capacitor because of the fluctuation in the inductance of the output transformer. This inability to match the resonant frequency of the tank circuit leads to lower operating efficiency. In addition, where rectangular-wave control signals enter from the external device, the voltage applied to the load becomes the differential of the rectangular waves, applying high voltages temporarily to the load. Consequently, no optimum control is applicable to the load.